The present invention generally pertains to communication signal scrambling and descrambling systems utilizing encryption technology and is particularly directed to a system for reproducing in a descrambler a key signal that is unique to the descrambler and was used in encrypting a key signal that must be decrypted for use in descrambling a signal received by the descrambler.
In the prior art the unique key signal that is used in encrypting the key signal that must be decrypted is stored in a memory contained in the descrambler and is accessed from the memory for use in decrypting a received encrypted key signal intended for the descrambler. Typically a person in possession of the descrambler could ascertain the unique key signal by reading it from the memory. The fact that the unique key signal could be ascertained by a possessor of a descrambler presented a significant problem when it was proposed by the applicants of cross-referenced Application Ser. No. 498,800 to use key signal encryption techniques for controlling the distribution of scrambled television signals to subscriber of a television suscription service. Although only the unique key signals of paid-up subscribers would be used in controlling such distribution, it was apparent that if a paid-up subscriber could ascertain his unique subscriber key signal stored in the memory of his descrambler, such unique subscriber key signal could then be stored in the memory of unauthorized descramblers to enable pirating of the scrambled television signals.
One solution to this problem suggested by the applicants of Application Ser. No. 498,800 was to use a secure memory in each descrambler for storing the subscriber key signal that is unique to the descrambler. A secure memory is an internal read only memory (ROM) in an integrated circuit microprocessor chip having an internal architecture that prohibits the ROM from being read off chip. The secure memory approach provides a high level of security against a person in possession of a descrambler ascertaining the unique subscriber key signal for such descrambler.
It is an object of the present invention to so complicate the reproduction of the unique subscriber key signal in the descrambler as to further lessen the probability of unauthorized ascertainment and use of such key signal.
It is a further object of the present invention to so complicate the reproduction of the subscriber key signal in such a manner as to make it practical for a plurality of broadcasters utilizing a common subscriber communication network to use subscriber key signals that are unique to both the individual subscriber and the individual broadcasters.